The Effects of Pilot Helmet, Posture, and Fatigue on Load Sharing in Cervical Extensors.

Abstract

Neck pain is highly prevalent in military fighter pilots and represents a significant threat to both individual health and operational readiness. Advanced helmet-mounted systems impose substantial mechanical demands on the cervical spine, increasing muscle activation and joint loading. These demands contribute to fatigue, tissue degeneration, and the development of chronic musculoskeletal disorders. Despite the recognized importance of muscle forces in spinal loading, the distribution of force among individual cervical muscles remains poorly understood due to the redundancy of the musculoskeletal system and limitations of traditional assessment techniques. Shear wave elastography has emerged as a promising method for estimating muscle mechanical properties in vivo that scale with both passive and active force production. Therefore, the overall goal of this work was to establish and apply shear wave elastography based methods to quantify individual muscle forces and examine how cervical muscle loading and load sharing are influenced by helmet configuration, posture, and fatigue. First, it was hypothesized that shear wave elastography derived estimates of individual muscle force in the triceps surae would correspond to externally measured joint torque and demonstrate good reliability within and across raters. The results showed that torque from the triceps surae closely corresponded to dynamometer-measured torque across multiple joint positions, supporting the validity of SWE as a method for estimating individual muscle force. Reliability measures indicated consistent force estimates across raters and testing sessions, demonstrating that SWE can provide stable and repeatable measurements of muscle force in vivo. Secondly, it was hypothesized that helmet mass and center of mass and non-neutral postures would impact cervical muscle forces. The results demonstrated that both helmet configuration and posture significantly influenced cervical muscle force production. Increased external flexion moments resulting from helmet mass and anterior center-of-mass placement required greater force generation from cervical extensors. These effects were further amplified in non-neutral postures, indicating an interaction between external loading and head/neck position in determining cervical muscle demand. Third, it was hypothesized that fatigue of the splenius capitis would reduce its force contribution and result in compensatory redistribution of load among synergistic cervical extensors. The results showed that fatigue reduced the relative force contribution of the splenius capitis; however, compensation was not uniform across synergistic muscles. Instead, load redistribution varied between individuals, with no single muscle consistently assuming increased load. Analysis of muscle coordination revealed that load sharing was organized through low-dimensional patterns of muscle co-activation, and that fatigue altered these patterns without producing consistent increases in any specific compensatory muscle. The findings of this dissertation demonstrate that cervical muscle loading and load sharing are highly dependent on external mechanical demands and neuromuscular state. SWE provides a viable method for estimating individual muscle forces in vivo, enabling direct investigation of muscle contributions to joint loading. Helmet configuration, posture, and fatigue each independently and interactively influence cervical muscle forces. These results provide new insight into the mechanisms underlying cervical spine loading in fighter pilots and highlight the importance of considering both mechanical and coordination-based factors in understanding and mitigating neck pain in high-load environments.